With model microbial systems, we plan to examine the ways that molybdenum is placed into certain molybdoproteins. Various molybdenum cofactors from nitrogenases as well as from milk xanthine oxidase will be analyzed with respect to stoichiometry and amino acid sequence. With the use of mutant strains and tungsten-grown cells of Azotobacter vinelandii and Klebsiella pneumoniae, we will examine the interrelationships between the molybdenum cofactors and other molybdenum-containing factors in the cells. Enzymes involved in these reactions will be partially purified. Molybdenum transport into the cell will be studied with respect to specific chelators and with membrane vesicles. A molybdenum storage protein will be purified and analyzed with respect to stoichiometry with molybdenum. In vitro transition of molybdenum from the storage protein to the molybdenum cofactors will be studied to determine the reactions involved. Regulation and genetics of molybdenum metabolism will be analyzed with the use of mutant strains and with purified genes in in vitro translation and transcription systems. The catalytic role of molybdenum cofactors will be studied by electron paramagnetic resonance spectroscopy.